Boron carbide as an effective friedel-crafts type catalyst

ABSTRACT

The compound, boron carbide, B 4 C, is an effective catalyst for the conversion of benzylic halides to polybenzyls.

CROSS REFERENCED TO RELATED APPLICATION

This application is a divisional of application Ser. No. 11/442,716 examined by J. Parsa.

BACKGROUND

Friedel-Craft alkylation and acylation reactions of organic compounds have been commonly performed with Lewis acid catalysts. However, the use of Lewis acid catalysts in commercial practice has presented problems of the catalysts being corrosive, difficult to recover and the generation of hazardous waste.

Examples of such reactions are described in the text by P. Bruice, Organic Chemistry, 4^(th) edition, Prentice Hall, 2004, pg. 612 and following. A common catalyst employed in both alkylation and acylation reactions is the Lewis acid AlCl₃. Although AlCl₃ is referred to as a catalyst in the true sense, it is not. It requires stoichiometric amounts of AlCl₃ since it actually forms a complex with the reactant that subsequently requires its removal from the reaction mixture by either an acid or base hydrolysis. Such a procedure is costly and, in the process, toxic waste is generated which must be disposed of. Other Lewis acid catalysts have been investigated which include zeolites as disclosed in U.S. Pat. Nos. 4,547,605 and 4,717,780. Although the zeolites are effective Lewis acid catalysts there use is often limited by the pore size of the zeolite which inhibit large sterically hindered molecules from reaching the active site within the zeolite.

Numerous Lewis acid catalysts have been disclosed which include both transition and non-transition metals as disclosed in U.S. Pat. No. 4,414,406 and U.S. Pat. No. 6,184,418, however often the catalysts are difficult to prepare or exhibit chemical reactivity that limits their use. Numerous disclosures include the utilization of mixed catalysts as described in U.S. Pat. No. 5,750,455.

Although much effort has been made to develop more effective catalysts for both alkylation and acylation of organic compounds, there is a need for more effective catalysts that do not have the inherent problems of the ones currently employed. An ideal catalyst would be one that functions as a heterogenous catalyst, easily removed from the reactants and products, chemically and thermally stable, and readily available or easily prepared, and inexpensive.

SUMMARY OF THE INVENTION

I have discovered that boron carbide, B₄C is an effective catalyst for Friedel-Crafts type reactions. It is readily available, chemically and thermally stable, requires no pretreatment, and is easily recoverable from the reaction products.

Since B₄C functions as a heterogenous catalyst and is non-toxic, no hazardous waste is generated at the conclusion of the reaction. The use of B₄C requires no time consuming work up at the end of the reaction and can be reused without any regeneration or activation procedures. This discovery is unexpected since B₄C is regarded in the literature as compound that is essentially unreactive.

DETAILED DESCRIPTION OF THE INVENTION

The current literature teaches that catalysts for Friedel-Crafts reactions are classified as Lewis acid catalysts. Although many materials have been investigated as Friedel-Craft catalysts they are all recognized as Lewis acid type catalysts. These include zeolites, clays, heteropoly acids, and various metal halides.

I have discovered that the non-metal carbide, boron carbide, B₄C can function as a catalyst in alkylation of aromatic compounds, previously conducted by Lewis acid Friedel-Craft type catalysts. This is unexpected since boron carbide is regarded as a compound that has a high resistance to chemical attack.

Boron carbide in an extremely hard material whose melting point is 2450° C. It is commonly used as an abrasive in lapping applications and as a refractory. It is also known to be a neutron absorber and is use in the nuclear industry. There are no reports in the chemical literature that boron carbide exhibits any chemical or physical properties that would indicate that it would function as a catalyst. This unexpected discovery is surprising in regard to the teachings in the prior art.

I have discovered that boron carbide is suitable for the alkylation of aromatics to produce polybenzyls. The rate of the reaction depends on both the amount of boron carbide present, its particle size and the temperature at which the reaction is conducted. The reactions are carried out by contacting the corresponding benzyl halide in the presence of the boron carbide. These reactions proceed to completion at temperatures ranging from 80° to 160° C.

This reaction can easily be observed by heating 20 ml of benzyl chloride to about 120° C. in an evaporating dish. At the end of an hour no observable reaction has taken place. If, at this point, 0.10 gm of boron carbide is added to the benzyl chloride within minutes copious amounts of HCl is evolved and the benzyl chloride is transformed into a dark viscous mass, which is the polybenzyl product.

The following examples illustrate the embodiments of this invention, however, it is understood, that they are presented only for illustrative purposes and do not limit the scope of this invention.

EXAMPLE 1

A mixture of 12.6 gm (mole) benzyl chloride and 0.1 gm of boron carbide was heated in an evaporating dish, with continuous stirring to 120° C. Initially copious amounts of HCl gas evolved, ceasing in about two hours. The viscous mixture solidified on cooling. The solid material was dissolved in benzene and the catalyst was removed by filtration. The benzene was then removed at reduced pressure and the remaining viscous material was identified as polybenzyl from its infrared and NMR spectra.

EXAMPLE 2

A mixture of 7.4 gm (0.04 moles) of benzyl bromide and 0.2 gm of boron carbide in 175 ml of benzene was refluxed for 24 hours. The solution was allowed to cool and filtered to remove the boron carbide. The benzene was removed at reduced pressure and 5.4 gm of a solid product was recovered. It was identified as diphenylmethane by its infrared and NMR spectra. 

1. A process for the alkylation of benzylic halides to produce polybenzyls by contacting the benzylic halides in the presence of the catalyst boron carbide at a temperature of 100° C. to 180° C.
 2. The process in claim 1 in which the reacting benzylic halide is benzyl chloride.
 3. The process in claim 1 in which the benzylic halide is an alkyl substituted benzyl chloride. 